Telluro- and selenopyryliums, even those very similar in structure to those described herein, have been known for use as dyes in photoconductive, photoresist and lithographic compositions or optical recording discs. See, e.g., U.S. Pat. Nos. 4,365,017 and 4,584,258, respectively. However, none of such descriptions recognized that the seleno- or telluropyrylium dyes might have a medicinal use, let alone one that is particularly effective in photodynamic therapy. To that end, the kinds of solvents and "carriers" taught for use with these known telluropyryliums have been biologically hazardous or deadly materials such as dichloromethane and bisphenol polycarbonates.
Meanwhile, the field of photodynamic therapy (pdt) has developed through the selective retention of certain dyes in localized, differentiated cancers such as differentiated carcinomas and melanoma, prior to exposure of this portion of the host to selected light energy. It has been found that dyes particularly effective in such treatment generate singlet oxygen or a superoxide anion, as reported in J. Clinical Oncology, Vol. 6, p. 380 (1988) and others. Because the dyes are selectively retained over time by the cancer cells, and not by the healthy cells, the ptd effect is selective also. Dyes that have been particularly recommended for this process include acridines, methylene blue, rhodamine 123, eosin, tetracycline, chlorophylls, and porphyrins such as hematoporphyrin derivatives, hereinafter HPD. The difficulty with all these conventional dye compositions used for photodynamic therapy, hereinafter pdt, has been that they absorb light primarily at wavelengths that are considerably shorter than 700 nm. In other words, they are relatively ineffective when exposed to wavelengths above 700 nm. Yet, it is well-known that light with wavelengths between about 700 and 1200 nm will readily pass through most biological tissues, unlike light of shorter wavelengths. As a result, it has been difficult to use pdt with cancers not readily accessible from the surface of the host. (Those that operate at 650 nm are still useful for surface treatment.)
Thus, prior to this invention there has been a substantial need to find cytotoxic, photoactivatable dyes that absorb primarily above 700 nm, for use i pdt.